Meristems are central to higher plant development, as almost all post-embyronic organs, including roots, leaves, flowers and axillary meristems, are initiated by either shoot or root meristems. Two aspects of shoot meristem development are central to its function. First, the meristem must maintain a group of cells in the center that remain undifferentiated. The proliferation of these undifferentiated meristem cells is necessary to provide new cells for organ initiation. In Arabidopsis, for example, new organs are generated throughout the life span, requiring the plant to maintain a pool of undifferentiated cells from which to draw for organ and new meristem initiation. Second, as the meristem apex moves away from these undifferentiated cells by continuted cell division, the undifferentiated cells that are now on the flanks of the meristem must be allowed to enter a specific developmental pathway, such as leaf or flower development, leading to eventual differentiation. It is the balancing of these two features that allows for the continued growth of the meristem and its continuous initiation of new organs.
Proliferation of meristem cells balanced with their subsequent incorporation into organ primordia relates to the classical separation of the shoot meristem into a central zone (CZ) and surrounding peripheral zone (PZ). The CZ at the very tip of the shoot meristem, would correspond to the region where cells are maintained in an undifferentiated state. In the PZ, daughter cells enter a specific developmental pathway and become incorporated into organ primordia. Molecular evidence corroborates this distinction. A number of mutants that disrupt meristem development have been described in Arabidopsis.
The existence of CLAVATA1 is well known in Arabidopsis thaliana. It was identified through the phenotype of its mutant alleles, best described in Clark et al., Development 119: 397-418 (1993); see also Koomneef et al., J. Hered. 74: 265-272 (1983); Bowman et al., Oxford Rev. Plant Cell & Mol. Biol. 5: 57-87 (1988); and Leyser et al., Development 116: 397-403 (1992). The gene has also been described under the names FLO5, FAS3 and FUF by rediscoverers, between the time when they found new alleles, and the time when it was shown that they had identified alleles of the earlier-described CLAVATA1 gene.
Mutations in the gene cause a loss of normal control of cell division in shoot apical meristems and in floral meristems. The result of loss of cell division control is in either case an enlargement of the meristem. In shoot apical meristems this enlargement can lead to fasciation (cresting), where the meristem grows very large, causing the stem to become straplike, and leaves and flowers to be produced in great profusion. In flowers the enlargement leads to an increase in the number of floral organs, including an increase in carpel number, which increases fruit size and seed number.
There are naturally occurring mutations whose phenotypes resemble those of CLAVATA1 in Arabidopsis. Celosia, or cock's comb, is a cultivated flowering plant that differs from its wild progenitors by fasciation, which enlarges the inflorescence. Since the inflorescence leaves (bracts) are the colorful part, the cultivated variety is much showier than the wild ones. Crested cactuses and other succulents also are considered highly desirable, but as cresting occurs in cactus as a rare response to unknown environmental conditions, the commercial crested cactuses are not genetic variants. Similarly, wild tomatoes are small berries with two carpels, or fruit components. The large, beefsteak-type cultivated tomatoes are mutants in a gene that gives extra floral organs, similar to CLAVATA1 mutants. The organs of interest are carpels, which are increased to about 6 in number, giving a much larger fruit.
A separate locus, CLAVATA2, appears to exhibit similar mutant phenotypes based on brief descriptions (McKelvie, Radiation Botany 1: 233-241 (1962); Koomneef et al., J. Hered. 74: 265-272 (1983)).
Thus, it would be desirable to be able to control the size and appearance of shoot and floral meristems, to give increased yields of leaves, flowers, and fruit. Accordingly, it is an object of the invention to provide for clavata1 nucleic acids and proteins, and modified clavata1 nucleic acids and proteins which result in altered meristem phenotypes.
It is a further object to provide plant cells and plants which contain the recombinant clavata1 nucleic acids and proteins of the invention.